Frustrated bilayer smectic phase in main-chain polymers with two different spacers

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Frustrated bilayer smectic phase in main-chain polymers with two different spacers

Junji Watanabe, Yasukazu Nakata, Kazuya Simizu

To cite this version:

Junji Watanabe, Yasukazu Nakata, Kazuya Simizu. Frustrated bilayer smectic phase in main-chain polymers with two different spacers. Journal de Physique II, EDP Sciences, 1994, 4 (4), pp.581-588.

�10.1051/jp2:1994149�. �jpa-00247984�

Classification

Physics

Abstracts

61.30 61.30E

Frustrated bilayer smectic phase in main-chain polymers with two different _spacers

Junji Watanabe,

Yasukazu Nakata

(*)

and

Kazuya

Simizu

Department of

Polymer Chemistry, Tokyo

Institute of

Technology, Ookayama, Meguro-ku, Tokyo

152,

Japan

(Received

10 December 1993, revised 26

January

1994,

accepted

18

February1994)

Abstract. We have studied the structural characteristics of the smectic

phases

formed

by

the main-chain type ^of

polymers

in which the

mesogenic biphenyl

moieties _are linked

by

two different odd-nuInbered

aliphatic

_spacers in _a

regularly

alternate fashion. The first

interesting

result of this

study

is that the two different types of

layer

structures _are formed

depending

_on the difference in the

lengths

of two

aliphatic

_spacers. In the

polymers

with the two spacers

having

_a sInall difference in their

lengths,

the

single layer

structure is formed _{as a} result of the random

mixing

of the two different spacers. In contrast, if there is

a

sufficiently large

difference in

lengths,

the two different spacers

participate

into different

layers

and _so the sInectic

phase

appears to have _a

bilayer

character. The second

interesting

result is that the

bilayer

smectic

phase pretends

the frustrated structure with the unusual

density1nodulation

in the direction

parallel

to the

layer.

This frustrated structure has been

explained

_as

resulting

from _a two-

dimensional _escape of the spontaneous

polarization.

We have _so far

reported

the

thermotropic mesophase properties

of main-chain BB-n

polyes-

ters that _can be constructed

by

_an alternate

arrangement

^{of the}

mesogenic p,p'-bibenzoate

unit and the flexible

alkylene

_spacer with the carbon number of _n 11-?]

1(~§~OiCH~(Ol

o o

n =

3 ~-9

This

homologous

series of BB-n

(n

=

3-9) invariably

form smectic

mesophases

whose

isotropiza-

tion

temperatures

^and

entropies

exhibit _an even-odd oscillation with the number of

intervening methylene units,

n. The even-odd oscillation _was also observed _on the

types

^of smectic

phases.

In even-membered BB-n there is formed _a normal smectic A

phase

in which both _axes of the

polymer

chain and

mesogenic

_group lie

perpendicular

to the

layers.

In contrast, ^{the smectic}

(*)

Present Address: LINTEC Co.,

Nishikicho, Warabishi,

Saitarna 335,

Japan.

582 JOURNAL DE

PHYSIQUE

II N°4

structure in odd-membered _{ones was} identified _{as a new}

type

^of smectic

phase,

smectic

CA [8],

in which the tilt direction of

mesogenic

_groups ^{is invariable} in every second

neighboring layer

but

opposite

to the other between

neighboring layers (refer

to

Fig.

6 of Ref.

[2]).

^{The forma-}

tion of such _a distinct

SCA

structure _as well _as the odd-even _appearance of smectic structures have been

explained

_as

resulting

from _a

coupling

of the

polymeric

and

mesogenic

^effects in which the

spatial arrangement

^of

mesogenic

_groups is

strongly

confined

by

the conformation of the

intervening alkylene

_spacers

[2, 4, 7, 9].

In addition _to this conformational constraint

effect,

another

interesting

^effect _can ^be consid- ered _on the smectic structure in the main-chain

type

of

polymers [10].

^{If there} are two different

alkylene

_spacers within _a

polymer

molecule which _are

incompatible sterically,

^and if there is

sufficient lateral attraction between identical spacers of

adjacent polymer molecules,

_segrega-

tion into

bilayer

_{may occur.}

Then,

four

possible types

^of

bilayer

structures will arise _{as a} result of _a

coupling

with the conformational constraint mentioned above. These _are illustrated in

figures

la to Id. The

bilayer

structures of

figures

la and 16 may be formed from the

polymers

in which both different _spacers have _even number of carbons. The

bilayer

smectic

phase

in

figure

lc may arise in the

polymers

with the two spacers different in _an odd-even

parity

of their carbon numbers.

Finally,

the

bilayer phase

of

figure

Id _can be

expected

for the

polymers

with the two odd-numbered _spacers.

a

b

c

d

Fig.

1. Four

possible

smectic _structures with the

bilayer

modifications which may be formed in the main-chain

polymers

with the two different _spacers

sequenced

in _a

regularly

alternate fashion.

Among these,

the

bilayer

smectic

phase

of

figure

id is

especially interesting

since it should be ferroelectric _even in nonchiral

system.

^The

ferroelectricity

_can be understood

by considering

the _space group. As illustrated in

figure 2,

the _space group is

analogous

to the

crystallographic C]~;

^there _are two fold _axes

along y-axis

and mirror

planes perpendicular

to z- and _z-axes

[11, 12].

^{Since there is} no mirror

plane perpendicular

to the two fold _axes, the spontaneous

polarization

_can be

expected

to arise

along

the

y-axis, I-e-,

in _a tilted direction in _a

layer [13].

The

present study

has been

performed

to seek this

type

of ferroelectric smectic

liquid crystal

in the

following

main-chain

polymers

in which _two

mesogenic biphenyl

moieties dud two odd- numbered

aliphatic

chains _are

sequenced

in _a

regularly

alternate fashion

[14].

1§~O(CH~(O~§-OfCH2§o

o o

n =

3, 5, 7, 9,11,13

U

-

i

~ ~

z z z

ihY ihY /~'~

~ ~

~

~

~ ~~ z

~~~

Fig.

2.

Space-group

symmetry for the

bilayer

smectic structure of

figure

id

ii11.

Here,

the

aliphatic

chain

containing

three

methylene

units is connected to the

mesogenic

_cores

by

two ester

linking

_groups, while the other

aliphatic

chain is linked to the

mesogenic

cores

by

two ether groups

containing

_a variable odd number of

methylene

_groups with _n

=

3, 5, 7, 9,

11 and 13. The

polymers

_were

designated

BP-C03-0n where 3 and _n indicates the

number of

methylene

_groups in the spacers. These _were

prepared by

_a

polycondensation

of the

corresponding 4,4'-dicarboxy-a,uJ-diphenylphenoxyalkane

and

1,3-propanediol,

which have been described elsewhere

[14].

^All the

polymers

form the smectic

mesophases.

The

phase

transition

temperatures

^and

enthalpies,

collected from the DSC

cooling

_{curves, are} listed in table I.

Depending

_on the

compatibility

of two flexible _spacers, two

possible types

^of smectic struc-

tures _can be considered to be formed. One is the

single-layer

smectic

phase

in which the _two

different _{spacers are}

compatible

and _so

randomly

mixed. Another is the

bilayer

smectic

phase

with the different spacers

segregated

from each other. These two

types

^of

layer

structures _can be

easily distinguished

from each other

through

the x-ray observations. In random

mixing

system,

the first-order

layer

reflection should arise with _a

spacing corresponding

to half the

length

of _a repeat ^{unit of}

polymer (L/2)

while in the

bilayer system

it arises with _a

spacing corresponding

to the

length

of _a repeat ^unit

(L).

Figure

3a shows the

X-ray

diffraction

pattern

^{observed for the oriented smectic}

phase

of BP- C03-07.

Here,

the oriented

specimen

has been

prepared

_{as a} fiber

by pulling

_up the

isotropic

584 JOURNAL DE

PHYSIQUE

II N°4

Table I. Phase transitions

(°C)

of BP-C03-0n

polymers

based _on

cooling

DSC

data;

K

=

crystal; Sm

=

smectic;

^I =

isotropic melt;

₌ transition

enthalpies (AH)

in

kJ/mol

of

repeat

unit.

/~i

-1<.>

~

~~~~

jansitious

~inh ~ _~

BP-C03-03 0.43 169

(4.6)

248

(13.4)

BP-C03-05 0.41 155

(4.6)

249

(18.4)

BP-C03-07 0.54 161

(7.5)

236

(17.1)

BP-C03-09 0.38 149

(3.3)

213

(18.0)

BP-C03-011 0.58 145

(3.8)

202

(19.2)

BP-C03-013 0.37 157

(3.8)

196

(23.4)

(*)Inherent

viscosities _were determined at 25 °C

by using

0.5

dlg~~

solutions in _a

60/40 w/w

mixture of

phenol

and tetrachloroethane.

melt and the fiber axis

corresponding

to the

polymer

chain axis is

placed

in the vertical direction

[2]. ^{The similar}

X-ray pattern

_was ^observed for

BP-C03-03,

BP-C03-05 and BP-C03-09. For

these

polymers

with _n

= 3 to

9,

^the first-order

layer

reflections

(the

001

reflections)

_appear

having

the

spacings

^of12.9

^Ji

to 16.1

Ji

as listed in table II. These values

approximate

to half the

repeat length (L/2)

_11,

_7], meaning

that the

layer

structure is constructed

by

_a random

mixing

of the two different _spacers.

Further,

it is found from

figure

3a that the

layer

^reflection

irises just

_{on a} meridian while the _outer broad reflection

showing

the

liquid-like

association of

mesogenic

_groups in _a

layer

_are

placed

above and below the

equator.

^This diffraction

geometry

clarifies the

SCA type

of

layer

structure

[2, 4].

Table II. The

spacing Ji

of

layer

reflections observed for the smectic

phases

of BP-C03-0n

polymers; ),

^{indices of} reflections

(see

the

text).

BP-C03-03 BP-C03-05 BP-C03-07 -C03-011 -C03-013

21.9

(101)

24.7

(101)

12.9

(001)

13.9

(001)

14.9

(001)

16.1

(001)

17.1

(002)

18.0

(002)

6.43

(002)

6.94

(002)

7.45

(002)

8.54

(004)

9.04

(004)

6.66

(105)

5.70

(006)

The

remarkably

distinct

X-ray

_pattern is observed for the smectic

phases

of the BP-Co3-011 and

BP-Co3-013,

as shown in

figure

3b. The _narrow distribution of reflection intensities also

points

to the

high degree

of orientation in fibers. An

interesting aspect

^{of this} pattern ^is that

a)

b)

Fig.

3.

X-ray

diffraction

photographs

of the oriented smectic

phases

observed in

(a)

BP-C03-07 and

(b)

BP-C03-011 fibers at 200 °C. The fiber

specimen

was

prepared by pulling

_up the

isotropic

melt and its axis is

placed

in the vertical direction. The

enlarged photograph

in

(b)

shows the

X-ray

pattern in the

small-angle region.

the meridional

layer

reflection is

accompanied by

four

point (off-meridional)

reflections

(see enlarged photograph

in

Fig. 3b).

The relative intensities of these reflections _are maintained

constant with the variation of

mesophase temperatures, showing

that these _come from _a

single

phase.

As listed in table

II,

the

spacing

of the first meridional reflection

corresponds

to half the

length

of

repeat

^unit

IL /2)

while the off-meridional reflections _appear with the

height

^of

1IL

far from the

equator.

This dictates that there is _a

density

modulation with _a

periodicity

of L

along

the

layer

normal. The

X-ray photograph

shows

additionally

weak but distinct

higher-

order reflections _on meridian and

off-meridian,

which _are also listed in table II. All of these reflections _can be

interpreted

_as ool

11 ⁼

2m)

and lot ₁₁

=

2m+1)

reflections based _on the twc-dimensional

rectangular

^{lattice. The lattice}

parameters

are determined _{as a}

= 28.5

Ji

_and

c = 34.2

Ji

_for

BP-Co3-011,

and _a

= 33.8

Ji

and _c

= 36.2

Ji

_for BP-Co3-013.

Here,

the c-axis

corresponds

to the

polymer

chain axis and a-axis lies

perpendicularly

to the

polymer

chain.

Considering

that the

averaged

diameter of molecule is _{more or} less than 4.5

Ji,

six to

eight

586 JOURNAL DE

PHYSIQUE

II N°4

molecules _are located in these lattices. The diffuse outer reflections _are also observed

here, lying

above and below the

equator. Hence,

the

mesogenic

_{groups are}

liquid-like

in _a

layer

with their _axes

alternately

tilted to the

layer

normal

similarly

_as in the

SCA Phase

of the random

mixing system.

^{Overall results show that the} smectic

phase pretends

^the

bilayer

structure of

figure

id

although

it also has another

periodic density

modulation in _a direction

parallel

to the

layer irrespective

of the

liquid-like

lateral association of

mesogenic

_groups. We thus _{come to a}

preliminary

conclusion that _even the

compatible aliphatic

_{spacers can} form different

layers

if there is _a

sufficiently large

^difference

(8

_{or more} in the carbon

number)

in their

lengths.

Let _us consider the

layer

structure

responsible

for

showing

the distinct

X-ray pattern

of

figure

3b. This characteristic

pattern containing

the meridional and off-meridional

layer

reflections is reminiscent of those

reported

for the frustrated smectic

phases [15-17]. Using

the

concept

developed

for the frustrated smectic

phases,

_{we can}

tentatively

_{propose a} structural model _as illustrated in

figure 4, according

to which the internal structure of the smectic

layers displays

an unusual

density

modulation

parallel

to the tilt direction of

mesogenic

_groups

[18].

The unusual

density

modulation is

produced

in such _{a way} that the

bilayer

is constructed

by

_a

repeat

^{unit of}

polymer

chain in _a small domain but the

polymer

chains in

adjacent

small

domain slide

halfway along

the

layer

normal after the 180° rotation around their chain _axes.

In other

words,

^it can be described

by

_a

periodic

structure of domain

walls,

in each domain the basic

layer

structure

being

the _{same as} the

bilayer

structure of

figure

id. Observation of _a

series of reflections with ₌ to 5 dictates the

highly positional

order

along

the

layer

normal

(c-axis)

while the

positional

order

along

the

layer la-axis)

is low because of the observation of reflections with

only

h

= 1. This shows that the

density

modulation

along

the

layer

is not

necessarily regular. Furthermore,

_{one can} find that the tilted orientation correlation of the

mesogenic

_groups, which should be

required

in the

liquid crystalline field,

has been maintained

irrespective

of the frustration

(see

dashed

curves).

c=3~.2i

S'

'11gJ

~p~J

'W

'~~

~>°

'

Fig.

4. Tentative structural model for the frustrated smectic

phase

of BP-C03-011. For convenience, the

polymer

chains in

an all-trans conformation

are illustrated.

The frustrated smectic

phases

with _a

twc-dimensionally positional

order have been

firstly

observed for low molar _mass

compounds

such _as the derivatives of

cyanobiphenyl possessing

a

strong longitudinal dipole [15].

Frustration effects arise in such _a

system

because of the

incommensurability

of two

types

of characteristic

lengths, namely,

the molecular

length

and the

pair length,

the

pair

formation

being

controlled

by dipolar

forces

[16, 17].

^{The frustrated}

smectic

structure,

_{as an} another

example,

has been observed in the combined main-chain

/side-

chain

polymers.

It has been

explained

to be caused

by

the combined orientation of side-chain and main-chain _mesogens

[19, 20], showing

that the frustrated smectic

phase

_can be induced

by

_an

appropriate design

of

polymer

molecule. In the

present system,

_can one make _sense for the

driving

force for the

adoption

of such _a frustrated smectic

phase?

If the fundamental

bilayer

smectic

phase

of

figure

id is

ferroelectric,

it turns out that the _answer is rather

simple.

By

_a

comparison

with the ferroelectric

bilayer phase

^of

figure id,

_{one can} find that there is

a twc-dimensional _escape of

polarization, _I-e-,

_an antiferroelectric

arrhngement

of

polarization

in the frustrated smectic

phase

of

figure

4. It is thus

postulated

that the frustration has been induced _to eliminate the

strong

interaction of the

spontaneous polarization. Any

^other _reasons

can be

hardly

considered since the frustration effects have _never been observed in the other

types

^of

bilayer

smectic

phases

of

figures

16 and 1c which have _no

spontaneous polarization [14].

^The

present

frustrated smectic

phase

with the

twc-dimensionally positional

order is the

third

example

which is caused

by

the different _reason.

References

iii

Watanabe J. and

Hayashi M.,

Macromolecules 21

(1988)

278.

[2] ^{Watanabe J. and}

Hayashi M.,

Macromolecules 22

(1989)

4083.

[3] Watanabe

J., Hayashi M.,

Kinoshita S. and Niori T.,

Polym.

J.

(Japan)

24

(1992)

597.

[4] Watanabe J. and Kiuoshita

S.,

J.

Phys.

II FFance 2

(1992)

1237.

[5] ^Takanishi Y., Takezoe

H.,

Fukuda A. and Watanabe

J., Phys.

Rev. B 45

(1992)

7684.

[6] ^Watanabe J., Komura H. and Niori

T., Liq. Cryst.

13

(1993)

455.

[7] ^Watanabe J.,

Hayashi M.,

Morita A. and Niori

T.,

Mol.

Cryst. Liq. Cryst.,

in press.

[8] ^{In the}

previous

reports, this

phase

has been termed SC2

Phase,

but

SC2

is _a

confusing

notation

in the context of the nomenclature described in low molar

mass systems. ^{For this} _{reason, we} will hereafter

adopt

the notation of

SC

_A which has been

adopted

for the

same type ^{of smectic}

phase

in low molar _mass system

by

Fukuda _et al.; Chandani

A-D-L-,

Gorecka

E.,

Ouchi

Y.,

Takezoe H. and Fukuda A.,

Jpn

J.

Appl. Phys.

28

(1989)

L1265.

[9] ^Abe

A.,

Macromolecules17

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2280.

[10] ^{Petscheck R-G- and}

Wiefling

K-M-,

Phys.

Rev. Let. 59

(1987)

343.

[11] International Table for X-ray

Crystallography (1959)

Vol. I

(Birmingham, Kynoch Press)

p. 208.

[12] ^{It should be noted here that this} space group is

essentially

different from that of the

SCA Phase

which is

analogous

to the

crystallographic D(~

[4, 11].

[13]

Meyer R-B-,

Mol.

Cryst. Liq. Cryst.

40

(1977)

33.

[14] Nakata Y., Shimizu K. and Watanabe J., to be

published.

[15]

Sigaud

G., Hardouin F., Achard M-F- and Levelut A-M-, J.

Phys.

France 42

(1981)

107.

(16] Prost J. and Barois P., J. Chim.

Phys.

80

(1983)

65.

[17] ^Prost

J.,

Adv.

Phys.

33

(1984)

1.

588 JOURNAL DE

PHYSIQUE

II N°4

[18] ^{It should} noted here that the fundamental

bilayer

smectic phase has _a biaxial character and _so another frustrated structure _can be also

proposed

such that the unusual

density

modulation

occurs in _a direction

perpendicular

to the tilt direction. In the present time, we cannot select

one of these two models since the

X-ray

pattern ^has been observed

only

for the fiber

specimen.

However, _even if _any model is

adopted,

the structural characteristics described here _are not

essentially

altered.

[19] ^Endres

B-M-,

Ebert

M.,

Wendorff

J.H.,

Reck B. and

Ringsdorf H., Liq. Cryst.

7

(1990)

217.

[20]

Mensinger H.,

Biswas A. and Poths H., Macromolecules 25

(1992)

3156.